My advice is similar to others. My daughter was baptized in the same gown my brother and I wore, as well as one of my cousins, my mom and aunt. We haven't done anything special to preserve it beyond washing it right after being used and keeping it in a closet or cedar chest. The little jacket my great grandmother made to go with it is slightly yellowed, but it actually has taken on kind of an ivory color that looks natural and still goes well with the gown.
To pull in both sides of the family, we actually used the little cap my husband wore for his baptism as well for my daughter. It's the kind that's actually made out of a hankerchief and was designed to cut the ribbons for him to give to his wife on his wedding day. Well, my mother-in-law didn't find it until 4 years into our marriage, so needless to say, it wasn't used for that purpose. But we ended up finding a special use for it anyway to use for his daughter's baptizm.